Maybe it's a rogue shinobi from another village who has a vendetta against the Hidden Leaf. They see the revival of the Hokages as an opportunity to take on the Hidden Leaf at its strongest. This antagonist would be cunning and powerful, using various tactics to try and undermine the Hokages and Naruto.
There's also a possibility that the main antagonist is an internal threat within the Hidden Leaf itself. A group of ninjas who oppose the revival of the Hokages for their own reasons, perhaps they fear a change in the power structure of the village. They could start causing chaos and try to stop Naruto from using the Hokages' help to protect the village.
It could be a new and powerful enemy that the past Hokages once sealed away. With Naruto's revival of the Hokages, this enemy somehow gets released or starts to break free from its prison. This antagonist would be extremely powerful, having been a threat that required the combined efforts of the Hokages to deal with in the past.
Well, one possible plot development could be that Naruto revives the Hokages to seek their wisdom in a new and dangerous threat that the village is facing. Maybe it's a threat that requires the combined knowledge and skills of all the past Hokages.
There could be a group of powerful out - of - world beings who are interested in the chakra of the Hokages. They revive the Hokages themselves, but with the intention of stealing their chakra. They are extremely powerful and have abilities that the ninja world has never seen before. Their arrival shakes the very foundation of the ninja world and forces the Hokages and Naruto to come up with new strategies to defeat them.
Kronos himself could be the main antagonist. Even if Percy revives him with good intentions, Kronos' nature might be too power - hungry and evil to be controlled.
It could be someone from the Naruto universe who doesn't like Naruto's newfound fascination with Aku. For example, Danzo might see Naruto's interest in Aku as a threat to the stability of the village and try to stop Naruto from further exploring Aku - related things. So, Danzo could be the main antagonist in this case.
A group of rogue ninja who are hunting nekoshou for their special powers could be the main antagonists. They see Naruto's newfound connection with the nekoshou as a threat and try to eliminate him and the nekoshou he is protecting.
It could be a corrupt ninja from within the village who wants Naruto out of the way. Maybe someone jealous of Naruto's popularity and power, so they frame him and get him sent to prison. This antagonist would then try to keep Naruto in prison by any means necessary.
Madara Uchiha could be a great antagonist. His powerful Sharingan abilities and his grand plans for the Naruto world would put him at odds with Supergirl. He might see her as a threat to his control and try to manipulate or defeat her. Another option could be Kaguya Otsutsuki. Her god - like powers and desire to reshape the world would clash with Supergirl's values and her attempts to protect the people of the Naruto universe.
Maybe it's a ninja who has also been affected by Anodite magic but has turned to the dark side. This ninja could be jealous of Naruto's new powers and want to take them for himself. He would use his knowledge of both ninja techniques and Anodite powers to try and defeat Naruto.
It could be Orochimaru. He's always been interested in immortality and reanimation techniques. Maybe he reanimates Naruto for some evil purpose, like using Naruto's power to take over the ninja world.
Maybe the Xenomorph Queen herself could be the main antagonist. She is intelligent and powerful. She sees Naruto's world as a new place to colonize and sends her Xenomorph minions to attack. Naruto has to find a way to stop her before she spreads too many eggs and creates an unstoppable army.